Two kinds of vacuum gauges are known: thermionic emission vacuum gauges (also called hot cathode vacuum gauges), and field emission (or cold cathode) vacuum gauges.
In thermionic emission vacuum gauges, the electron source consists in one or more filaments, for instance of tungsten, brought to incandescence. A typical example of hot cathode vacuum gauge is the Bayard-Alpert vacuum gauge. That kind of vacuum gauge comprises a wire-shaped ion collecting plate, a cylindrical grid surrounding said plate and an incandescent tungsten filament for electron emission, located outside the grid. The electrons emitted by the filament and accelerated by the grid ionise the residual gas, and the ions and/or the ionised positive molecules are collected by the plate, which is kept at lower potential than the electron source and the grid.
In the disclosed design, the electrons pass several times through the grid and, during such in and out movement, they ionise the residual gas until they hit the grid and are absorbed by it.
Due to that design including a plate reduced to a simple wire, pressures as low as about 10−9 Pa could be measured. Because of the reduced plate wire surface, the background current is minimised due to photoelectric effect from the plate (electron emission) caused by X rays produced by electrons hitting the grid.
Such a vacuum gauge is disclosed for instance in U.S. Pat. No. 2,605,431, “Ionisation Vacuum Gauge”.
The major drawback of that kind of vacuum gauges is related to the nature of the electron-emitting cathode. Actually, a heated filament is an isotropic electron source, whereas directionality of the electron beam is an essential parameter for vacuum gauge sensitivity.
In the disclosed design, vacuum gauge sensitivity is not constant, since the distribution of the electron emission direction changes as the temperature along the emitting cathode filament changes, said filament typically reaching temperatures up to about 2000° C.
Moreover, the phenomenon of electron emission by thermionic effect entails high power consumption, long response times and a non-negligible pollution of the surrounding environment due to the release of impurities.
In order to improve the performance of Bayard-Alpert vacuum gauges, use of more recent technologies has been proposed for making the electron-emitting cathode.
U.S. Pat. No. 5,278,510 “Ionisation Vacuum Gauge Using a Cold Micropoint Cathode” discloses a vacuum gauge wherein, in order to obviate the drawbacks mentioned above, the heated filament is replaced by a microtip cold cathode.
A microtip cathode comprises a set of very small tip-shaped emitters, located beneath an extraction grid provided with corresponding openings. The microtips are made of metal (molybdenum, niobium) or of silicon-polysilicon and are placed on supports of the same material, or of silicon or glass. By applying a potential difference between the microtips and the extraction grid, an electric field is produced that is strong enough to produce electron emission. This microtip cathode is an extremely directional electron source, with low consumption and very short response time.
FIG. 1 schematically shows a microtip disclosed above. On a plane substrate 1, for instance of niobium or molybdenum, a tip 3 of the same metal, about 1 μm high, is grown. An extraction grid 5 is located above substrate 1 and parallel thereto, and has openings 7 in correspondence with each tip 3. Said openings 7 typically have diameters of about 1 μm. Said grid 5 is kept separated from said substrate 1 and from tips 3 by an insulating layer 9, having a cavity 11 in correspondence with each tip 3 to allow electron emission from tip 3 through the corresponding opening 7 in extraction grid 5.
Usually, the microtips are produced in arrays and adjacent microtips are spaced apart by few micrometers, so that densities of the order of 106 to 108 microtips/cm2 can be achieved.
The teaching of U.S. Pat. No. 5,278,510 provides, however, only a partial response to the problems inherent in ionisation vacuum gauges.
More particularly, the vacuum gauge described above, which reproduces the geometry of a conventional Bayard-Alpert vacuum gauge, is cumbersome and difficult to use in many applications. Moreover, said vacuum gauge is by itself a non-negligible distortion in pressure measurement, taking also into account the high vacuum degrees it is intended for.
This is mainly due to the fact that, even if the microtip cathode is a highly directional electron source, electron movement into and out of the cylindrical grid does not allow reducing the vacuum gauge size.